1. Field of the Invention
The present invention relates to a current-mode control device and a switching power supply having a current-mode control device, and more particularly to a slope compensation circuit used in a current-mode control device that is built into a current-mode type DC-DC converter, a current-mode type switching power supply or the like.
2. Description of the Related Art
FIG. 5 shows a switching power supply having a current-mode control device with a slope compensation circuit. The switching power supply is disclosed in Japanese Examined Utility Model Publication No. 7-39346 shown and is of an isolating type with a transformer 1, in which the switching on/off action of a switching element 2 (switching element constructed of a MOS-FET in the example in FIG. 5) connected to the primary coil N1 of the transformer 1 induces a voltage in the secondary coil N2 of the transformer 1 based on the input voltage supplied by a DC power supply 3, and the voltage induced in the secondary coil N2 is rectified and smoothed by a rectifying and smoothing circuit 4 to output an output voltage V.sub.out to a load.
To output stably the predetermined output voltage V.sub.out, the switching power supply has a current-mode control device 5 that switching-controls the switching element 2 in a current-mode control.
As shown in FIG. 5, the current-mode control device 5 includes a current transformer 6 for detecting a switching current flowing through the switching element 2, a resistor 7 is connected in parallel with the output side of the current transformer 6, the anode of a diode D1 is connected to the output of the current transformer 6, the cathode of the diode D1 is connected to one terminal of a resistor 8, and the other terminal of the resistor 8 is grounded. The node X of the diode D1 and the resistor 8 is connected to a predetermined input terminal of a switching control circuit 10.
The node X of the diode D1 and the resistor 8 is also connected to one terminal of a resistor 11, the other terminal of the resistor 11 is respectively connected to the anode of a diode D2, one terminal of a resistor 12, and one terminal (input side) of a capacitor C1, the cathode of the diode D2 is connected to the other terminal of the resistor 12, and the other terminal of the capacitor C1 is grounded.
The node Y of the diode D2 and the resistor 12 is connected to the node Z of the emitter of an NPN transistor 13 and the emitter of a PNP transistor 14. The collector of the transistor 13 is connected to a power supply V.sub.cc, and the collector of the transistor 14 is grounded. The bases of both transistors 13, 14 are connected to each other, and the node of these bases is connected to an output terminal of the switching control circuit 10. The node Z of the emitter of the transistor 13 and the emitter of the transistor 14 is connected to the gate of the switching element 2.
The non-inverting input terminal of an operational amplifier 15 is connected to the output of the rectifying and smoothing circuit 4, the inverting input terminal of the operational amplifier 15 is connected to a DC power supply V1, and the output of the operational amplifier 15 is connected to a predetermined input terminal of the switching control circuit 10.
A current sense circuit 16 is constructed of the current transformer 6, resistors 7, 8 and diode D1. A slope compensation circuit 18 is constructed of resistors 11, 12, diode D2, and capacitor C1. A drive circuit 19 is constructed of the NPN transistor 13 and PNP transistor 14. An output voltage detector circuit 20 is constructed of the operational amplifier 15 and DC power supply V1.
The switching control circuit 10 includes a circuit arrangement that switching-controls the switching element 2 to regulate the output voltage V.sub.out by outputting a rectangular wave pulse signal having a switching period of T as shown in FIG. 6A and variably controlling the on period t of the pulse signal through current-mode control.
The current-mode control device 5 is thus constructed, and the operation of the current-mode control device 5 is briefly discussed here. When the on signal is supplied by the switching control circuit 10, the transistor 13 of the drive circuit 19 is turned on, and the transistor 14 of the drive circuit 19 is turned off, and a drive voltage is applied from the power supply V.sub.cc to the gate of the switching element 2 of the drive circuit 19 via the transistor 13, thereby turning on the switching element 2. A switching current I.sub.sw flowing through the switching element 2 during the switch-on period increases at a gradient of ml of a linear straight line as shown by a full line A in FIG. 7, the switching current I.sub.sw is then detected and converted into a voltage by the current sense circuit 16, and the current sense circuit 16 outputs a voltage V.sub.sp represented by a dotted line in FIG. 6C or the full line A in FIG. 7, having a gradient identical to the gradient ml of the switching current I.sub.sw of the switching element 2.
During the switch-on period of the switching element 2, a current flows from the power supply V.sub.cc, to the capacitor C1 through the transistor 13 and resistor 12, charging the capacitor C1. The voltage at the capacitor C1 rises at a gradient of the straight line of a linear function with time as shown in FIG. 6B. The voltage at the capacitor C1 is output across the resistor 11 as the output of the slope compensation circuit 18, and the output of the slope compensation circuit 18 is then superimposed on the output voltage V.sub.sp of the current sense circuit 16, thereby compensating for the slope of the output voltage V.sub.sp of the current sense circuit 16, and a voltage V.sub.s/ represented by the full line in FIG. 6C is then applied to the switching control circuit 10.
The operational amplifier 15 detects the output voltage V.sub.out of the switching power supply, amplifies the differential voltage between the output voltage V.sub.out and the voltage of the power supply V1, and outputs to the switching control circuit 10 a voltage V.sub.op responsive to the output voltage V.sub.out, and the switching control circuit 10 thus determines the timing of switching off of the switching element 2 to regulate the output voltage V.sub.out through current-mode control based on the output voltage V.sub.s/ of the current sense circuit 16 and the output voltage V.sub.op of the operational amplifier 15.
The voltage V.sub.s/ of the current sense circuit 16 increases as represented by the full line in FIG. 6C subsequent to the switch-on of the switching element 2, and when the voltage V.sub.s/ reaches the output voltage V.sub.op of the operational amplifier 15, the switching control circuit 10 outputs an off signal. For example, when the output voltage V.sub.out is lower than a preset voltage, the output voltage V.sub.op of the operational amplifier 15 rises and the time required for the voltage V.sub.s/ to reach the V.sub.op of the operational amplifier 15 is lengthened, and the on period of the switching element 2 gets longer, thereby compensating for a drop in the output voltage. Conversely, when the output voltage V.sub.out is higher than the preset voltage, the output voltage V.sub.op of the operational amplifier 15 falls and the time required for the voltage V.sub.s/ to reach the V.sub.op of the operational amplifier 15 is shortened, and the on period of the switching element 2 gets shorter, thereby compensating for a rise in the output voltage. The switching control circuit 10 controls the on period of the switching element 2 in this way, regulating the output voltage V.sub.out.
When the off signal is output by the switching control circuit 10, the transistor 13 is turned off, the transistor 14 is turned on, the switching element 2 is switched off, the diode D2 is turned on, and the voltage at the capacitor C1 is discharged through the diode D2 and transistor 14 to be ready for charging for a next switch-on cycle.
As described above, the switching power supply shown in FIG. 5, comprising the current-mode control device 5, reliably outputs the preset output voltage V.sub.out by switching-controlling the switching element 2 through current-mode control using the current-mode control device 5.
It is known that a duty factor (the ratio of the on period t to the switching period T (t/T)) of 50% or higher of the pulse signal output by the switching control circuit 10 helps increase circuit efficiency of the switching power supply. At a duty factor of 50% or higher, however, a low-frequency oscillation takes place, destabilizing the operation of the switching power supply.
The current-mode control device 5 disclosed in the above-cited Japanese Examined Utility Model Publication No. 7-39346 is provided with the slope compensation circuit 18 to restrict low-frequency oscillations, and the gradient (slope) of the voltage V.sub.sp is compensated for by superimposing the output voltage of the slope compensation circuit 18 onto the output voltage V.sub.sp of the current sense circuit 16 so that the voltage V.sub.s/ greater than the voltage V.sub.sp in gradient is fed to the switching control circuit 10, and the generation of the low-frequency oscillations is restricted even at a duty factor of 50% or higher and the operation of the switching power supply is thus stabilized.
The reason for this is as follows. For example, when the preset output voltage V.sub.out is reliably output by the switching power supply having the current-mode control device 5 without slope compensation circuit 18 shown in FIG. 5, the voltage V.sub.sp from the current sense circuit 16 fed to the switching control circuit 10 changes in a sawtooth voltage waveform shown by the full line A in FIG. 7 in accordance with the change in the switching current I.sub.sw of the switching element 2.
More particularly, the voltage V.sub.sp takes the following voltage waveform, wherein, when the switching element 2 is turned on by the switching control circuit 10, the output voltage V.sub.sp of the current sense circuit 16 rises and reaches a voltage V.sub.on, for example, and the output voltage V.sub.sp of the current sense circuit 16 increases at a gradient m1 of the straight line of a linear function with time during the switch-on period of the switching element 2, and when the switching element 2 is turned off by the switching control circuit 10, the output voltage V.sub.sp of the current sense circuit 16 drops to zero, and when the switching element 2 is turned on next time, the output voltage V.sub.sp rises to the voltage V.sub.on determined by a gradient m2 represented by a dot-dash line B during the switch-off period shown in FIG. 7.
When the output voltage V.sub.sp of the current sense circuit 16 rises to a voltage deviating from the voltage Von with the switching element 2 turned on, the voltage V.sub.sp increases, as shown by a dotted line D, at a gradient identical to the gradient ml represented by the full line A in FIG. 7, and the output voltage value of the current sense circuit 16 at the rise of the next switch-on is determined by a dot-dash line C having a gradient identical to the gradient m.sub.2 represented by the dot-dash line B, and with the slope compensation circuit 18 omitted, a deviation .DELTA.V1 of the output voltage V.sub.sp of the current sense circuit 16 from the voltage V.sub.on at the turn-on of the switching element 2 is expressed as .DELTA.V.sub.1 =(m.sub.2 /m.sub.1).multidot..DELTA.V.sub.0 where .DELTA.V.sub.0 designates the preceding deviation.
When the duty factor is 50% or higher, the absolute value of the gradient m.sub.1 of the voltage V.sub.sp, during the switch-on period, applied to the switching control circuit 10 is smaller than the absolute value of the gradient m.sub.2 represented by the dot-dash line during the switch-off period ( i.e., .vertline.m.sub.2 /m.sub.1 .vertline.&gt;1). Thus, the deviation of the output voltage V.sub.sp of the current sense circuit 16 from the voltage V.sub.on increases each time the switching element 2 is switched on (i.e., .DELTA.V.sub.1 &gt;.DELTA.V.sub.0), failing to converge and generating low-frequency oscillations, and once the switching power supply runs out of its stable operational state, it remains unable to revert back to its stable operation state due to the low-frequency oscillations, leading to an unstable operation of the switching power supply.
For this reason, the circuit shown in FIG. 5 includes the slope compensation circuit 18, and compensates for the gradient (slope) of the output voltage V.sub.sp of the current sense circuit 16 during the switch-on period of the switching element 2, using the slope compensation circuit 18 as shown in FIG. 6C and attempts to avoid generating the low-frequency oscillations by applying to the switching control circuit 10 the voltage V.sub.s/ having a gradient greater than the gradient ml of the output voltage V.sub.sp of the current sense circuit 16.
The applicants of the present application produced the switching power supply shown in FIG. 5 on an experimental basis, and found that an insufficient restriction of the low-frequency oscillations possibly led to an unstable operation of the switching power supply.
The present invention has been developed with a view to resolving the above problem, and the object of the present invention is to provide a current-mode control device that restricts low-frequency oscillations and stabilizes the circuit operation of a switching power supply even if the switching control of a switching element is performed at a duty factor of 50% or higher.